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What's the purpose of “true” in bash “if sudo true; then”

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17

I've hacked together this bash script which tests if the user has superuser privileges and if they don't it asks for them. Ultimately I'm trying to invert the second 'if' statement so that I can remove the following two lines (the echo "password ok" and the else on the following line)

# Root user only
if [[ "$EUID" != 0 ]]; then
 sudo -k # make sure to ask for password on next sudo
 if sudo true; then
 echo "Password ok"
 else
 echo "Aborting script"
 exit 1
 fi
fi

echo "do my ops"

Is the purpose of "true" on the fourth line just a null-statement?

I need to invert the test on the fourth line, how do I do so? Here is what I've tried:

if sudo false; then 
if sudo true == false; then
if [!(sudo true)]; then

bash sudo

share|improve this question

edited 13 hours ago

Matt Parkins

asked yesterday

Matt ParkinsMatt Parkins

19717

New contributor

Matt Parkins is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  If you're going to downvote the question, please could you explain what I can do to improve the question? Is this not the right overflow site?
  
  – Matt Parkins
  yesterday
  
  

  
  
  
  


• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  Not sure where the downvotes are coming from; are you trying to negate the statement or nullify the statement? You say you want to get rid of both the true/echo and the "else", so what's your final objective?
  
  – Jeff Schaller♦
  yesterday
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  see stackoverflow.com/q/10552711/537980
  
  – ctrl-alt-delor
  yesterday
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @ctrl-alt-delor following that link it seems all I need do is put an exclamation mark in front of the sudo and then I can remove both the echo and the else, and now i've tested it, that works, thanks.
  
  – Matt Parkins
  yesterday
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  In passing, you probably ought to redirect the error message to the error stream: echo "Aborting script" >&2
  
  – Toby Speight
  yesterday
  
  

  
  
  
  

 | 
show 7 more comments

17

I've hacked together this bash script which tests if the user has superuser privileges and if they don't it asks for them. Ultimately I'm trying to invert the second 'if' statement so that I can remove the following two lines (the echo "password ok" and the else on the following line)

# Root user only
if [[ "$EUID" != 0 ]]; then
 sudo -k # make sure to ask for password on next sudo
 if sudo true; then
 echo "Password ok"
 else
 echo "Aborting script"
 exit 1
 fi
fi

echo "do my ops"

Is the purpose of "true" on the fourth line just a null-statement?

I need to invert the test on the fourth line, how do I do so? Here is what I've tried:

if sudo false; then 
if sudo true == false; then
if [!(sudo true)]; then

bash sudo

share|improve this question

edited 13 hours ago

Matt Parkins

asked yesterday

Matt ParkinsMatt Parkins

19717

New contributor

Matt Parkins is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  If you're going to downvote the question, please could you explain what I can do to improve the question? Is this not the right overflow site?
  
  – Matt Parkins
  yesterday
  
  

  
  
  
  


• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  Not sure where the downvotes are coming from; are you trying to negate the statement or nullify the statement? You say you want to get rid of both the true/echo and the "else", so what's your final objective?
  
  – Jeff Schaller♦
  yesterday
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  see stackoverflow.com/q/10552711/537980
  
  – ctrl-alt-delor
  yesterday
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @ctrl-alt-delor following that link it seems all I need do is put an exclamation mark in front of the sudo and then I can remove both the echo and the else, and now i've tested it, that works, thanks.
  
  – Matt Parkins
  yesterday
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  In passing, you probably ought to redirect the error message to the error stream: echo "Aborting script" >&2
  
  – Toby Speight
  yesterday
  
  

  
  
  
  

 | 
show 7 more comments

I've hacked together this bash script which tests if the user has superuser privileges and if they don't it asks for them. Ultimately I'm trying to invert the second 'if' statement so that I can remove the following two lines (the echo "password ok" and the else on the following line)

# Root user only
if [[ "$EUID" != 0 ]]; then
 sudo -k # make sure to ask for password on next sudo
 if sudo true; then
 echo "Password ok"
 else
 echo "Aborting script"
 exit 1
 fi
fi

echo "do my ops"

Is the purpose of "true" on the fourth line just a null-statement?

I need to invert the test on the fourth line, how do I do so? Here is what I've tried:

if sudo false; then 
if sudo true == false; then
if [!(sudo true)]; then

bash sudo

share|improve this question

edited 13 hours ago

Matt Parkins

asked yesterday

Matt ParkinsMatt Parkins

19717

New contributor

Matt Parkins is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

# Root user only
if [[ "$EUID" != 0 ]]; then
 sudo -k # make sure to ask for password on next sudo
 if sudo true; then
 echo "Password ok"
 else
 echo "Aborting script"
 exit 1
 fi
fi

echo "do my ops"

if sudo false; then 
if sudo true == false; then
if [!(sudo true)]; then

share|improve this question

edited 13 hours ago

Matt Parkins

asked yesterday

Matt ParkinsMatt Parkins

19717

New contributor

Matt Parkins is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

share|improve this question

edited 13 hours ago

Matt Parkins

asked yesterday

Matt ParkinsMatt Parkins

19717

New contributor

Matt Parkins is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked yesterday

Matt ParkinsMatt Parkins

19717

New contributor

Matt Parkins is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked yesterday

Matt ParkinsMatt Parkins

19717

2 Answers
2

active

oldest

votes

36

true in bash isn't a keyword, it's a program that instantly exits with a successful exit code.
Likewise, false is a program that exits with an unsuccessful exit code.

You can try this out by running both programs from your terminal, and then reading the $? variable, which contains the exit code of the last program;

true
echo $? # 0
false
echo $? #1

if sudo true isn't equivalent to if sudo == true. if sudo true is running the true program using sudo, and checking the exit code.

Therefore:

if sudo false; then is running the program false as sudo. The return will always be false.

if sudo true == false will run the program true with the arguments == and false using sudo. This obviously isn't want you intended.

if [!(sudo true)] is invalid syntax.

What you are probably looking for is

if ! sudo true;

share|improve this answer

answered yesterday

JShorthouseJShorthouse

50728

New contributor

JShorthouse is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

• 
  
  
  

  
  10
  

  
  
  
  
  
  

  
  
  As a slightly pedantic correction to the first sentence: in bash, true and false are "builtins", commands interpreted directly by the shell; in Unix-like systems in general, they also exist as standalone programs on the file system. The difference doesn't matter that much here, but as far as I know, sudo true will be running the standalone program, not the bash builtin.
  
  – IMSoP
  yesterday
  
  

  
  
  
  


• 
  
  
  

  
  30
  

  
  
  
  
  
  

  
  
  PS: my favourite summaries of true and false are the titles on their man pages: true - do nothing, successfully and false - do nothing, unsuccessfully
  
  – IMSoP
  yesterday
  
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @IMSoP That's deep.
  
  – brainplot
  12 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @IMSoP I'm curious, what's the point of having shell builtins that exactly duplicate the behavior of some programs? I'm tempted to guess performance, but I'd not expect true to ever be called nearly often enough to matter.
  
  – John Dvorak
  11 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  If true didn't exist, the code in the question shows why one might need to invent it! It's a minimal program that can only fail if the system it's on is truly borked. So one can use it to establish if one's access credentials are OK.
  
  – nigel222
  7 hours ago
  

  
  
  
  

 | 
show 4 more comments

17

I feel like the accepted answer didn't actually answer your question?

The purpose of doing this is to check that you can actually sudo.

How this check is performed is via the true program as explained in the accepted answer.

share|improve this answer

answered yesterday

MehrdadMehrdad

1,37641534

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Essentially, this. Alternative could be to check if the user is in sudoers group, but just running sudo true is also easy enough of a way, although a bit hacky.
  
  – Sergiy Kolodyazhnyy
  21 hours ago
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  Note that this also requires sudo to be installed, so the checks wouldn't be entirely equivalent... you could be in the sudoers group and still not be able to sudo. (This can especially come up if you're e.g. writing a script to setup a new image from a minimal rootfs tarball, which might not have sudo.)
  
  – Mehrdad
  21 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  The OP wants to drop out if you cannot sudo, and get rid of two lines rather than have an else following a noop.
  
  – mckenzm
  21 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  And, conversely, you can also be able to sudo without being in the sudoers group if you (either personally or as a member of some other group) are listed in /etc/sudoers. But even sudo true is not infallible, as sudo access can be granted for only specific commands, so you may be able to sudo true, but not to sudo something_else, or vice-versa. There's no iron-clad way to test other than by trying to run the command you actually want to run with sudo.
  
  – Dave Sherohman
  10 hours ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Also, you can be in the sudoers group and not be able to sudo because the sudoers group is not listed in the sudoers file. A group with the four letters sudo in its name is not a valid indication of whether or not a user in that group can do any particular thing or anything at all using sudo.
  
  – jrw32982
  6 hours ago
  

  
  
  
  

 | 
show 1 more comment

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36

true in bash isn't a keyword, it's a program that instantly exits with a successful exit code.
Likewise, false is a program that exits with an unsuccessful exit code.

You can try this out by running both programs from your terminal, and then reading the $? variable, which contains the exit code of the last program;

true
echo $? # 0
false
echo $? #1

if sudo true isn't equivalent to if sudo == true. if sudo true is running the true program using sudo, and checking the exit code.

Therefore:

if sudo false; then is running the program false as sudo. The return will always be false.

if sudo true == false will run the program true with the arguments == and false using sudo. This obviously isn't want you intended.

if [!(sudo true)] is invalid syntax.

What you are probably looking for is

if ! sudo true;

share|improve this answer

answered yesterday

JShorthouseJShorthouse

50728

New contributor

JShorthouse is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

• 
  
  
  

  
  10
  

  
  
  
  
  
  

  
  
  As a slightly pedantic correction to the first sentence: in bash, true and false are "builtins", commands interpreted directly by the shell; in Unix-like systems in general, they also exist as standalone programs on the file system. The difference doesn't matter that much here, but as far as I know, sudo true will be running the standalone program, not the bash builtin.
  
  – IMSoP
  yesterday
  
  

  
  
  
  


• 
  
  
  

  
  30
  

  
  
  
  
  
  

  
  
  PS: my favourite summaries of true and false are the titles on their man pages: true - do nothing, successfully and false - do nothing, unsuccessfully
  
  – IMSoP
  yesterday
  
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @IMSoP That's deep.
  
  – brainplot
  12 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @IMSoP I'm curious, what's the point of having shell builtins that exactly duplicate the behavior of some programs? I'm tempted to guess performance, but I'd not expect true to ever be called nearly often enough to matter.
  
  – John Dvorak
  11 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  If true didn't exist, the code in the question shows why one might need to invent it! It's a minimal program that can only fail if the system it's on is truly borked. So one can use it to establish if one's access credentials are OK.
  
  – nigel222
  7 hours ago
  

  
  
  
  

 | 
show 4 more comments

36

true in bash isn't a keyword, it's a program that instantly exits with a successful exit code.
Likewise, false is a program that exits with an unsuccessful exit code.

You can try this out by running both programs from your terminal, and then reading the $? variable, which contains the exit code of the last program;

true
echo $? # 0
false
echo $? #1

if sudo true isn't equivalent to if sudo == true. if sudo true is running the true program using sudo, and checking the exit code.

Therefore:

if sudo false; then is running the program false as sudo. The return will always be false.

if sudo true == false will run the program true with the arguments == and false using sudo. This obviously isn't want you intended.

if [!(sudo true)] is invalid syntax.

What you are probably looking for is

if ! sudo true;

share|improve this answer

answered yesterday

JShorthouseJShorthouse

50728

New contributor

JShorthouse is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

• 
  
  
  

  
  10
  

  
  
  
  
  
  

  
  
  As a slightly pedantic correction to the first sentence: in bash, true and false are "builtins", commands interpreted directly by the shell; in Unix-like systems in general, they also exist as standalone programs on the file system. The difference doesn't matter that much here, but as far as I know, sudo true will be running the standalone program, not the bash builtin.
  
  – IMSoP
  yesterday
  
  

  
  
  
  


• 
  
  
  

  
  30
  

  
  
  
  
  
  

  
  
  PS: my favourite summaries of true and false are the titles on their man pages: true - do nothing, successfully and false - do nothing, unsuccessfully
  
  – IMSoP
  yesterday
  
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @IMSoP That's deep.
  
  – brainplot
  12 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @IMSoP I'm curious, what's the point of having shell builtins that exactly duplicate the behavior of some programs? I'm tempted to guess performance, but I'd not expect true to ever be called nearly often enough to matter.
  
  – John Dvorak
  11 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  If true didn't exist, the code in the question shows why one might need to invent it! It's a minimal program that can only fail if the system it's on is truly borked. So one can use it to establish if one's access credentials are OK.
  
  – nigel222
  7 hours ago
  

  
  
  
  

 | 
show 4 more comments

true in bash isn't a keyword, it's a program that instantly exits with a successful exit code.
Likewise, false is a program that exits with an unsuccessful exit code.

You can try this out by running both programs from your terminal, and then reading the $? variable, which contains the exit code of the last program;

true
echo $? # 0
false
echo $? #1

if sudo true isn't equivalent to if sudo == true. if sudo true is running the true program using sudo, and checking the exit code.

Therefore:

if sudo false; then is running the program false as sudo. The return will always be false.

if sudo true == false will run the program true with the arguments == and false using sudo. This obviously isn't want you intended.

if [!(sudo true)] is invalid syntax.

What you are probably looking for is

if ! sudo true;

share|improve this answer

answered yesterday

JShorthouseJShorthouse

50728

New contributor

JShorthouse is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

true
echo $? # 0
false
echo $? #1

if ! sudo true;

share|improve this answer

answered yesterday

JShorthouseJShorthouse

50728

New contributor

JShorthouse is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

answered yesterday

JShorthouseJShorthouse

50728

New contributor

JShorthouse is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

answered yesterday

JShorthouseJShorthouse

50728

17

I feel like the accepted answer didn't actually answer your question?

The purpose of doing this is to check that you can actually sudo.

How this check is performed is via the true program as explained in the accepted answer.

share|improve this answer

answered yesterday

MehrdadMehrdad

1,37641534

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Essentially, this. Alternative could be to check if the user is in sudoers group, but just running sudo true is also easy enough of a way, although a bit hacky.
  
  – Sergiy Kolodyazhnyy
  21 hours ago
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  Note that this also requires sudo to be installed, so the checks wouldn't be entirely equivalent... you could be in the sudoers group and still not be able to sudo. (This can especially come up if you're e.g. writing a script to setup a new image from a minimal rootfs tarball, which might not have sudo.)
  
  – Mehrdad
  21 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  The OP wants to drop out if you cannot sudo, and get rid of two lines rather than have an else following a noop.
  
  – mckenzm
  21 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  And, conversely, you can also be able to sudo without being in the sudoers group if you (either personally or as a member of some other group) are listed in /etc/sudoers. But even sudo true is not infallible, as sudo access can be granted for only specific commands, so you may be able to sudo true, but not to sudo something_else, or vice-versa. There's no iron-clad way to test other than by trying to run the command you actually want to run with sudo.
  
  – Dave Sherohman
  10 hours ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Also, you can be in the sudoers group and not be able to sudo because the sudoers group is not listed in the sudoers file. A group with the four letters sudo in its name is not a valid indication of whether or not a user in that group can do any particular thing or anything at all using sudo.
  
  – jrw32982
  6 hours ago
  

  
  
  
  

 | 
show 1 more comment

17

I feel like the accepted answer didn't actually answer your question?

The purpose of doing this is to check that you can actually sudo.

How this check is performed is via the true program as explained in the accepted answer.

share|improve this answer

answered yesterday

MehrdadMehrdad

1,37641534

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Essentially, this. Alternative could be to check if the user is in sudoers group, but just running sudo true is also easy enough of a way, although a bit hacky.
  
  – Sergiy Kolodyazhnyy
  21 hours ago
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  Note that this also requires sudo to be installed, so the checks wouldn't be entirely equivalent... you could be in the sudoers group and still not be able to sudo. (This can especially come up if you're e.g. writing a script to setup a new image from a minimal rootfs tarball, which might not have sudo.)
  
  – Mehrdad
  21 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  The OP wants to drop out if you cannot sudo, and get rid of two lines rather than have an else following a noop.
  
  – mckenzm
  21 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  And, conversely, you can also be able to sudo without being in the sudoers group if you (either personally or as a member of some other group) are listed in /etc/sudoers. But even sudo true is not infallible, as sudo access can be granted for only specific commands, so you may be able to sudo true, but not to sudo something_else, or vice-versa. There's no iron-clad way to test other than by trying to run the command you actually want to run with sudo.
  
  – Dave Sherohman
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  Also, you can be in the sudoers group and not be able to sudo because the sudoers group is not listed in the sudoers file. A group with the four letters sudo in its name is not a valid indication of whether or not a user in that group can do any particular thing or anything at all using sudo.
  
  – jrw32982
  6 hours ago
  

  
  
  
  

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show 1 more comment

I feel like the accepted answer didn't actually answer your question?

The purpose of doing this is to check that you can actually sudo.

How this check is performed is via the true program as explained in the accepted answer.

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answered yesterday

MehrdadMehrdad

1,37641534

share|improve this answer

answered yesterday

MehrdadMehrdad

1,37641534

answered yesterday

MehrdadMehrdad

1,37641534

answered yesterday

MehrdadMehrdad

1,37641534